Metal–Metal Bonding in Tri‐Actinide Clusters: A DFT Study of [An₃Cl₆] ^z (z = 1–6) and [An₃Cl₆Cp3] ^z (z = −2– +3; An = Ac, Th, Pa, U, Np, Pu)

The actinide–actinide bonding in tri-actinide clusters [An₃Cl₆]ᶻ (An = Ac–Pu, z = 1–6) and [An₃Cl₆Cp₃]ᶻ (z =−2–+3; Cp = (η5-C5H5)) is studied using density functional theory. We find 3-centre bonding similar to the tri-thorium cluster [{Th(η⁸-C₈H₈)(μ₃-Cl)₂}₃{K(THF)₂}₂]∞, as we previously reported (Nature 2021, 598, 72–75). The population of 3-centre molecular orbitals (3c-MOs) by zero, one or two electrons correlates with shortening of the An–An bond lengths, which also decrease with increasing actinide atomic number, consistent with the contraction of the actinide valence atomic orbitals. Mulliken analyses indicate that these 3c-MOs predominantly involve An 6d and 5f orbitals. Various methods evidence the presence of An–An bonding in most systems with populated 3c-MOs, including bond orders (Mayer and Wiberg), quantum theory of atoms in molecules metrics (ρ, ∇²ρ, −G/V, H, delocalization indices), electron localization function, and electron density assessments. Additionally, we explore the effect of Cp ligand substitution on uranium complexes, finding that bulkier Cp ligands can induce U–U bond distortions and result in slightly longer U–U bonds. Overall, this study advances our understanding of metal–metal bonding in tri-actinide clusters, highlighting its effects on geometric and electronic structures